1. Field of the Invention
This invention relates to a rubber composition for an inner liner, and more particularly, to a rubber composition for an inner liner of a tubeless tire.
2. Description of Related Art
It is known that there are usually two types of tire structures for maintaining the inner pressure of an air-containing tire, that is, a structure composed of a tire and a tube not integrated with the tire, and a tubeless structure where a tire itself functions as a container for air.
Needless to say, the role of tube is to prevent escaping of air, so that not only airtightness at a joint of a tube and a valve, but also gas permeability of wall of the tube itself (inversely, airtightness) is an important factor.
The gas permeability is an inherent property of the polymer used. Practically speaking, there is not any polymer better than butyl rubber (isobutylene-isoprene rubber, IIR). Even at present, tubes are usually produced by using IIR as a main component.
"Inner liner" is a material adhered to the inside surface of a tire so as to maintain airtightness. In the early days, natural rubber and SBR were used as inner liners, but when they are used for a long period of time, air having permeated the liner is stored in carcass and thereby various problems occur concerning durability.
However, as it is known, it is difficult to adhere a good airtight butyl rubber to natural rubber and the like, and therefore, butyl rubber can not be easily used as an inner liner. In order to overcome the problem, a modified butyl rubber, that is, a halogenated butyl rubber has been used. This polymer has a gas permeability substantially similar to that of butyl rubber, and moreover, can be adhered to natural rubber and SBR. Therefore, the halogenated butyl rubber is one of the best materials as an inner liner for tubeless tires.
Since retention of the inner pressure is an important role for air-containing tires used as passenger car tires, track and bus tires and bicycle tires, a rubber composition comprising a halogenated butyl rubber as a main component is generally disposed at the inside of tire as an inner liner layer so as to keep the inner pressure.
As an example of a rubber composition for inner liner, Japanese Patent Application Laid-open No. Sho 57-172945 discloses a rubber composition for inner liner comprising 100 parts by weight of a rubber material prepared by blending 50-100 parts by weight of halogenated butyl rubber and 0-50 parts by weight of diene type rubber and 5-50 parts by weight of a resin homogenizing agent prepared by adding a small amount of ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer and/or ethylene-methacrylic acid copolymer to a petroleum resin.
Japanese Patent Application Laid-open No. Sho 57-195760 discloses a heat resistant rubber for an inner liner comprising 100 parts by weight of a rubber material composed of 10-80 parts by weight of a butyl rubber and 90-20 parts by weight of a diene rubber and 2-10 parts by weight of calcium oxide. This rubber aims at improvement in heat resistance.
Japanese Patent Application Laid-open No. Sho 62-62848 discloses a rubber composition for an inner liner having a specified storage dynamic modulus which comprises 100 parts by weight of a rubber composition composed of 60-100 parts by weight of halogenated butyl rubber and 0-40 parts by weight of diene rubber and a hydrocarbon oil type softening agent comprising 60% or more of a paraffin component and 5% or less of an aromatic component.
Japanese Patent Application Laid-open No. Sho 62-62849 discloses a rubber composition for inner liner comprising 100 parts by weight of a rubber composition composed of 60-100 parts by weight of a halogenated butyl rubber and 0-40 parts by weight of a diene rubber with which 0.5-10 parts by weight of a tert-butylphenol acetylene resin is compounded. This rubber composition for an inner liner exhibits a high stickiness to a tire case when not yet vulcanized, a high adhesion to a tire case when vulcanized, and good low temperature characteristics, and the formation of crack is suppressed.
As a rubber composition containing coumarone resin, Japanese Patent Application Laid-open No. Hei 1-215838 discloses a rubber composition for a tire tread which comprises 100 parts by weight of a diene rubber, 5-20 parts by weight of tung oil containing glycerides of eleostearic acid and oleic acid, 1-3 parts by weight of a triazine type accelerator and 1-30 parts by weight of a rosin type resin containing coumarone resin.
Japanese Patent Application Laid-open No. Sho 62-1735 discloses a tire tread rubber composition for race comprising 100 parts by weight of SBR containing 25-50 parts by weight of high styrene and 50 parts by weight of a coumarone-indene resin.
Halogenated butyl rubber can improve the inner pressure retaining property to a great extent as compared with other diene rubbers. However, the degree of shrinkage remarkably increases when non-vulcanized, and therefore, the processability is deteriorated, so that there is a problem in manufacturing.
For example, in the case of molding green tires (before vulcanizing), exfoliation is liable to occur at a part of an inner liner bonding portion forcibly self-adhered owing to the increased self-shrinking force. This is problematic in manufacturing.
In addition, after being formed into a liner by means of rolls or the like, the degree of shrinkage changes largely and therefore, there is a problem as to the accuracy of constitution parts. Further, it is difficult to form a halogenated butyl rubber into a thin film and therefore, the productivity is disturbed.
For improving the rubber compositions by making the shrinkage as small as possible, application of softners such as oils and the like has been tried, but this treatment disadvantageously lowers the inner pressure retaining property.
On the other hand, increase in the amount of a filler such as carbon black is effective to improve the anti-shrinking property and inner pressure retaining capacity, but disadvantageously lowers durability of tires, that is, early occurrence of cracks in tires upon running due to increase in modulus of elasticity of rubber and insufficient dispersion.
In view of such a situation as above, a straight chain hydrocarbon resin such as ESCOREZ 1102 is proposed as a compounding agent which does not relatively lower the running durability and can improve the anti-shrinking property and the improvement in anti-shrinkage is observed to some extent, but is not sufficient and the inner pressure retaining property is somewhat lowered though the degree of lowering is not so much as that caused by oils.
When conventional tackifiers such as phenolic resins, terpene resins and petroleum hydrocarbon resins are used, the results are similar to those as above.
In view of the foregoing, the present inventors have contemplated to solve the problems and completed the present invention.